SST dies are used in a wide variety of products and applications related to emitting and/or sensing radiation. Several types of SST dies that emit electromagnetic radiation in the visible light spectrum are used in mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, computers, tablets, and other portable electronic devices for backlighting and other purposes. SST dies are also used for signage, indoor lighting, outdoor lighting, vehicle lighting, and other types of general illumination.
FIG. 1A is a cross-sectional view of a conventional SST die 10a having lateral configuration. As shown in FIG. 1A, the SST die 10a includes an SST 40 on a growth substrate 17. The SST 40 can be an LED having a transduction structure 30 comprising an active material 15 between layers of N-type GaN 16 and P-type GaN 14. The active material 15 contains gallium nitride/indium gallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”). The SST 40 also includes a P-type contact 18 on the P-type GaN 14 and an N-type contact 19 on the N-type GaN 16. In operation, electrical power provided to the SST die 10a via the P-type and N-type contacts 18 and 19 causes the active material 15 to emit light.
FIG. 1B is a cross-sectional view of another conventional SST die 10b in which the P-type and N-type contacts 18 and 19 are in a vertical configuration. During formation of the SST die 10b, the N-type GaN material 16, active material 15 and P-type GaN material 14 are grown on a growth substrate (not shown in FIG. 1B), which can be similar to the growth substrate 17. After forming the transduction structure 30, a carrier 20 is attached to the P-type contact 18. For example, one side of the P-type contact 18 can be attached to the P-type GaN 14 and the other side of the P-type contact 18 can be attached to the carrier 20 using a bond material 22, which can be composed of metal or metal alloy layers. The bond material can be a Ni-Sn-Ni stack such that one Ni layer contacts the carrier 20 and the other Ni layer contacts the P-type contact 18. Other bond materials, such as CuSn and/or TiSi, can be used. Next, the growth substrate can be removed and the N-type contact 19 can be formed on the N-type GaN 16. The structure is then inverted to produce the orientation shown in FIG. 1B.
Most electronic devices and many other applications require a white light output. However, true white light LEDs are not available because LEDs typically emit light at only one particular wavelength. For human eyes to perceive the color white, a mixture of wavelengths is needed. One conventional technique for emulating white light with LEDs includes depositing a converter material (e.g., a phosphor) on an LED. For example, FIG. 2A shows a conventional lighting device 30a that includes a device substrate 50, an SST die 10a or 10b mounted on the device substrate 50, and a converter material 60 on the SST die 10a-b. The light emitted from the SST 40 undergoes at least partial conversion while passing through the converter material 60 as explained in more detail below with respect to FIG. 2C.
Multiple SST dies 10a-b can be used in a lighting device. For example, FIG. 2B is a cross-sectional view of a conventional multi-SST lighting device 30b having the device substrate 50, a plurality of SST dies 10a-b attached to the device substrate 50, and the converter material 60 over the SST dies 10a-b. The multi-SST lighting device 30b also has a single lens 80 over the SST dies 10a-b. Other conventional multi-SST lighting devices may have a dedicated lens per SST or a group of SSTs. All the SSTs 40 in the multi-SST lighting device 30b are typically connected to a common anode and cathode such that all of the SST dies 10a-b operate together.
FIG. 2C schematically illustrates the light frequency conversion and scattering/reflection in a conventional lighting device 30c in which the lens 80 encloses the SST 40 and the converter material 60. The SST 40 emits blue light (B) that can stimulate the converter material 60 to emit light at a different frequency, e.g., yellow light (Y). Some blue light (B) emitted by the SST 40 passes through the converter material without stimulating the converter material 60. Other blue light (B) emitted by the SST 40 stimulates the converter material 60, which, in turn, emits yellow light (Y). The combination of the emissions of blue light (B) from the SST 40 and the emissions of yellow light (Y) from the converter material 60 is designed to appear white to a human eye if the blue and yellow emissions are matched appropriately. However, not all light emitted by the SST 40 ultimately leaves the lighting device 30c. For instance, the converter material 60 scatters some blue light (B) back toward the SST 40. Additionally, some light that reaches the outer edge of the single lens 80 reflects back toward the converter material 60 and further toward the SST 40.
FIGS. 3A-3C show top views of several conventional lighting devices constructed as schematically illustrated in FIGS. 1B and 2A with different patterns of N-type contacts over the N-type GaN of the SSTs 40. The patterns of N-contacts are designed to distribute electrical current through the N-type GaN to other relevant parts of the SST. Some examples of the N-contact patterns are rail-type N-contacts running within the outline of the SST 40 (FIG. 3A), dot-type N-contacts distributed over the SST 40 (FIG. 3B), and rail-type N-contacts extending beyond the outline of the SST 40 (FIG. 3C). The N-contacts 19 can cover an appreciable percentage of the surface area of the SST 40. Since the N-contacts 19 may absorb a significant portion of the light that is scattered from the converter material, refracted from the lens 80, or reflected from other objects (not shown), the N-contacts 19 are relatively “dark regions” on the surface of the SST that reduce the output. Consequently, even though a particular N-contact layout may improve the distribution of electrical current through the SST 40, conventional N-contacts may impair the visual appearance and reduce the efficiency of the device.